As a method of forming a relief plate by forming projections and depressions on a surface, the method what is called “analog plate-making” is well known, in which, using a photosensitive elastomer composition or a photosensitive resin composition, the composition is exposed to ultraviolet light through an original film to selectively cure portions of the photosensitive resin composition that are to form an image, and uncured portions of the photosensitive resin composition are removed with developing solution.
The analog plate-making often requires an original film made of silver halide material, and accordingly, the time and cost to produce the original film are needed. In addition, developing the original film requires chemical treatment, and disposal of waste fluid from development is also required, which is disadvantageous in terms of environmental sanitation.
As a method for solving the problems involved in the analog plate-making, there is disclosed a resin letterpress printing plate precursor comprising a photosensitive resin layer and a laser-sensitive mask layer element provided thereon that is able to form an image mask in situ (see, for example, Patent Documents 1 and 2). The method of making such a plate precursor is a plate-making method in which laser radiation is performed based on image data controlled using a digital device, forming an image mask in situ from a mask layer element, after which, similarly to the analog plate-making, exposure to ultraviolet light through the image mask is performed, or a photosensitive resin layer and the image mask are developed away. This method is called “Computer-To-Plate (CTP) method” in the field of resin letterpress printing plates. The CTP method has solved the above-described problems involved in the process for producing an original film, but has a problem of disposal of waste fluid resulting from the development of the photosensitive resin layer.
As an alternative method for solving the problems involved in the developing process and the waste fluid from development, many kinds of plate-making by direct engraving with a laser, what is called “laser engraving” have been proposed. Laser engraving, which forms projections and depressions, i.e., a relief, by literally engraving with a laser, is advantageous in that the shape of the relief can be freely controlled unlike relief formation using an original film. For example, portions to reproduce a reverse text on a material to be printed may be engraved deeply, and portions to reproduce a fine dot can be engraved into a shape with shoulders so that the dot does not fall over under printing pressure.
Patent Documents 3 to 5 disclose a resin letterpress printing plate precursor that can be engraved by a laser, or a resin letterpress printing plate produced by laser engraving. Patent Document 8 discloses a resin composition for laser engraving containing a partially saponified polyvinyl acetate as a polymer and a polymerizable double bond.
The partially saponified polyvinyl acetate and derivatives thereof, however, are fragile upon repeated impacts during printing, causing a problem of, for example, crack occurrence in a relief during printing.
As a method for solving the above-described problem, addition of a polyvinyl acetate derivative and a polyamide having basic nitrogen is proposed, for example, in Patent Documents 6 and 7. This method, however, is still not at a satisfactory level regarding the crack occurrence in a relief upon repeated impacts during printing under pulling tension exerted when the relief is wrapped around a cylinder drum of a printer.
Also known is to introduce reactive groups used in a photosensitive resin composition for a printing plate into the side chain of a partially saponified polyvinyl acetate (Patent Document 8).